For the saturated carbonyl trimer Ru(3)(CO)(12) theoretical methods predict a doubly bridged Ru(3)(CO)(10)(mu-CO)(2) structure to lie only 0.3 kcal mol(-1) above the unbridged global minimum in accord with the fluxional properties observed experimentally by carbon-13 NMR on the very stable unbridged Ru(3)(CO)(12). For M(3)(CO)(11) and M(3)(CO)(10) the global minima for the Fe, Ru, and Os derivatives each have totally different arrangements of the carbonyl groups. Thus the global minimum of Ru(3)(CO)(11) is singly edge-semibridged Ru(3)(CO)(10)(mu-CO) in contrast to the doubly face-bridged Fe(3)(CO)(9)(mu(3)-CO)(2) and triply edge-bridged Os(3)(CO)(8)(mu-CO)(3) found as global minima for the iron and osmium analogues, respectively. Furthermore, comparison of our predicted nu(CO) frequencies for low-lying unbridged, singly bridged, and doubly bridged isomers of Ru(3)(CO)(11) with the 1987 experiments of Bentsen and Wrighton indicates that three different Ru(3)(CO)(11) isomers are generated in these low-temperature inert matrix Ru(3)(CO)(12) photolysis experiments depending on the conditions. For Ru(3)(CO)(10) the global minimum is a triply bridged structure Ru(3)(CO)(7)(mu-CO)(3), which is very different from the Fe(3)(CO)(9)(mu(3)-CO) and Os(3)(CO)(8)(mu(3)-CO)(mu-CO) global minima found for the iron and osmium analogues, respectively. For Ru(3)(CO)(9) the global minimum is a singly bridged structure Ru(3)(CO)(8)(mu-CO) analogous to the global minimum for the osmium analogue but totally different from the triply bridged global minimum of Fe(3)(CO)(9).